1. Industrial Field of Application
This invention relates to a substrate for the thermal head of a heat transfer printer or a thermal printer, an optical IC, an optical switch or the like, that is to say, a silicon substrate which can be favorably used as a substrate required to have both heat insulating property and heat radiating property and its production method, and more specifically to a silicon substrate having porous oxidized silicon layers which constitute the heat storage layers of a thermal head, the heat insulating layers of an optical switch or the like and its production method.
2. Description of the Prior Art
Drawing 18 indicates a conventional thermal head. This thermal head has an alumina substrate 1 and heat storage layers 2 made of glass glaze and formed on the alumina substrate 1. A heating resistor layer 3 is formed on the heat storage layer 2 and the substrate 1. A conductor layer 4 for feeding a current to the layer 3 is formed on the heating resistor layer 3. One surface of the substrate 1 is dotted with heating parts 6 each consisting of a conductor layer 4 and a resistor layer 3. A protective layer 5 for protecting the heating parts 6 from oxidation and abrasion is formed over them.
This thermal head is used while being pressed upon a recording medium such as an ink ribbon or a thermal paper (not shown in the drawing). The ink of an ink ribbon can be thermally transferred and a recording medium can be colored by heating the heating part 6 of the thermal head by feeding a current to the heating part 6.
Increasing the amount of heat stored in the heat storage layer 2 made of glass glaze by increasing its heat capacity by increasing its thickness is a means for improving the thermal efficiency of such a thermal head.
However, increasing the thickness of the heat storage layer 2 made of glass glaze results in a prolonged time required for its temperature drop after heating and deteriorated thermal response of the thermal head.
In order to improve the thermal efficiency of the thermal head while avoiding such a problem, it is recommended to provide heat storage layers having low thermal conductivity and small heat capacity.
Porous oxidized silicon (POS) is known as a material which can be used to form such heat storage layers. POS is highly heat resistant and has sufficient mechanical strength.
A thermal head having heat storage layers made of POS has been proposed in Japanese Patent Laid-open No. 257,652/1988. Referring to FIG. 19, this thermal head has a silicon substrate 11 and a heat storage layer 12 made of POS and formed over the entire one surface of the silicon substrate 11. On the heat storage layer 12 made of POS, a non-porous oxidized silicon layer 13, a heating resistor layer 3, a conductor layer 4 and protective layers 5 are formed in that order.
A known method for producing the silicon substrate 11 of this thermal head consists of subjecting a silicon substrate 11 to anodic formation in an electrolytic solution (aqueous hydrofluoric acid solution) composed of hydrofluoric acid, alcohol and water to form a layer consisting of porous silicon (PS) and then thermally oxidizing the PS layer.
In the above silicon substrate 11, however, since the volume of the PS layer is increased about 2.2 times during its thermal oxidation into POS, internal stress (compression stress) remains in the formed heat storage layer 12 thereby resulting in great warping of the silicon substrate 11.
In practical production of the above substrate 11, forming a POS layer (heat storage layer 12) with 25 .mu.m thickness results in 1 mm warping per inch of the substrate 11. This warping is about 10 times that of a commercially available alumina ceramic substrate. The above silicon substrate 11 was of no practical use because of such great warping.